The present invention relates to heating systems in which a mixture of a fuel vapor and air are burned flamelessly on the surface of a catalyst. The invention relates particularly to the rapid and efficient starting of the combustion of the fuel when both the fuel and the catalyst are initially cold; i.e., at a temperature near room temperature or lower.
The catalytic oxidation of hydrocarbons, alcohols, and other chemicals at elevated temperatures has been practiced efficiently for many years in industrial processes, and more recently in such applications as small space heaters fueled by propane. A characteristic of such applications is that they operate steadily for relatively long periods, and are started from an initially cold state only infrequently. Thus, the catalytic space heater can be ignited with a match, and large industrial catalytic systems can be brought slowly up to operating temperature by auxiliary heaters. Such systems would be more conveniently started by a self igniting catalyst.
For certain applications which might be most advantageously served by catalytic heaters, however, the heating periods are very short and the system must be heated frequently from a cold start; hence very rapid and efficient initiation of combustion is mandatory, and an auxiliary heating system for starting combustion is unacceptable. One such application is the heating of shave foam or cosmetics dispensed from an aerosol can, where a few grams of material must be heated from 50.degree. F. to 80.degree. F. in a period of 10 to 20 seconds, or less. In this case the heating system must be small and inexpensive, but capable of developing high heating intensities very quickly, and also capable of completely burning the fuel without production of noxious combustion products. It is also imperative that the system be highly efficient in converting and transferring the heating value of the fuel into the fluid which is to be heated.
Experience has shown, however, that suitable fuels, such as hydrocarbons and alcohols, when vaporized and mixed with air, will not start to burn spontaneously when brought into contact with catalysts of the types commonly used, unless the catalyst or the fuel, or both are preheated.
A further difficulty encountered in the use of conventional catalyst results from the fact that the catalytically active metal, for example platinum black, is most effective when supported upon highly porous base materials such as gamma alumina or silica gel, which are highly hygroscopic. Thus, between heating periods when the system is cool, the catalyst support material adsorbs moisture from the ambient air which greatly reduces the activity of the catalyst. This will completely block the spontaneous oxidation of preferred fuels such as methanol on the surface of the catalyst when both catalyst and fuel are initially cold.
It is also highly desirable that catalytic heating systems for applications such as those mentioned above use a fuel whose vapor pressure is higher than atmospheric so that a high velocity jet of fuel vapor can be used to aspirate the air required for combustion, thus avoiding the need for pumps or other pressurizing devices. Thus, pure methanol, one of the most easily catalytically oxidized fuels cannot be used without some pressure generating device. Hydrogen, another fuel which is readily oxidized catalytically, is not as practical in these applications because of the difficulties and cost of storing and handling this gas under high pressure.
Accordingly, objects of the present invention are to provide apparatus and a method for rapid, spontaneous ignition of catalytic combustion with a liquid fuel, preferably an alcohol, which is effective at low temperature relative to operating temperature and in the presence of moisture in the catalyst mass.
A further object is to provide catalytic combustion with high efficiency of fuel utilization and without production of noxious combustion products.